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PRACTICAL ORGANIC CHEMISTRY
determination may be made by introducing fresh pellets of
When the observations are complete, the apparatus is
allowed to cool and the weight of benzene ascertained by
weighing the boiling-tube and benzene.
As in the freezing-point method, the molecular weight is
calculated from the weight of substance required to raise the
boiling-point of ioo grams of solvent i°, and the result multiplied
by a coefficient which depends upon the nature of the solvent.
The following is a list of solvents commonly employed and
their coefficients and boiling-points :—'•
The molecular weight is determined from the formula
M - I0° kw
in which w is the weight of substance, W that of the solvent,
d the rise of boiling-point, and k the coefficient
Example.—Using the same solvent and adding successively
four pellets of naphthalene, the following results were
W. d M.
Calculated for C]0H8 ; M = 128.
A simpler and more convenient form of Beckmann apparatus,
requiring much less solvent and giving equally accurate results,
is shown in Fig. 33. It consists of a boiling-tube furnished with
two side pieces, one of which is stoppered and serves to
introduce the substance and the other acts as a condenser. The
boiling-tube stands on an asbestos pad and is surrounded by
two short concentric glass cylinders surmounted by a mica plate.
The other parts of the apparatus are similar to those in the older
form and the process is conducted in the same way.